Allow arch-specific stl selection

Test: m -j checkbuild
Change-Id: Ic55433e39b2a0108efcfe980503265ac139b22e2
diff --git a/cc/stl.go b/cc/stl.go
index 9e67145..65d7e40 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -23,7 +23,7 @@
 	// select the STL library to use.  Possible values are "libc++", "libc++_static",
 	// "stlport", "stlport_static", "ndk", "libstdc++", or "none".  Leave blank to select the
 	// default
-	Stl *string
+	Stl *string `android:"arch_variant"`
 
 	SelectedStl string `blueprint:"mutated"`
 }